This is an excerpt from the paper of Gustavo Costa et al., 2015. I am not sure what is the meaning of arbitrary units and how do I translate this to flow rate in volume per minute.
"Mass analyses were obtained in the negative ion mode. The mass spectrometer was programmed to perform three consecutive scans: full mass MS1 (m/z 160–1300),MS2 of the most abundant ion inMS1, and MS3 of the most abundant ion in MS2. Source
voltage was 4.5kV and capillary voltage and temperature were -10Vand250 °C, respectively. Nitrogen was used as sheath gas at flow rate of 20 arbitrary units. The normalized energy of collision was 45%, using helium as collision gas."
- Any values shown as "arbitrary", are by definition, relative units (that is what the word means).
I can understand how as a student, this might be confusing for someone like yourself with no formal training in LC-MS analysis. As scientists, we prefer specific values, but for some types of analysis, not all of the settings have value. Gas flow rates for a specific LC-MS application are instrument independent. They must be optimized to the specific instrument for the application. All of the LC-MS settings used must be optimized. Do not use the settings used by someone else as their instrument and lab are different. Any settings reported are just a guideline. No two instruments can be expected to produce the same results on the same sample with the same settings. Any gas flow rate settings used would only be as a general guide and specific only to the person(s) in the laboratory where there were used. IOW: They are relative. *This is common knowledge among scientists who develop methods on and use LC-MS instruments.
Different instruments can give different outputs as their signals. Then it is not easy to present them with a specific unit. Artibitrary units or relative units are used do that the magnitudes of one measurement is compared with other. Hope this will do.
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